Le strutture del Padiglione CINA: intervista allo studio Simpson Gumpertz & Heger
Il padiglione della Cina è sicuramente tra i più interessanti di EXPO 2015. Il tema che hanno scelto “the Land of Hope” (la terra della speranza), si manifesta pienamente anche nell’architettura del padiglione, in cui ambiente urbano e paesaggio si incontrano in un unico luogo.
Il progetto del padiglione cinese, concepito e sviluppato da un team multidisciplinare, è stato pensato per combinare le forme evocatrici del paesaggio con i materiali della tradizione costruttiva cinese, rivisitati ed interpretati attraverso il linguaggio dell’architettura contemporanea.
Per conoscere invece gli aspetti strutturali abbiamo intervistato lo studio Simpson Gumpertz & Heger, studio che si è occupato della progettazione strutturale del padiglione.
Q: What kind of structure did you design?.
A: To promote coexistence of city and nature in harmony, sharp-edged, angled timber rafter members shape the back of the pavilion’s roof, and gentle, soft and curvy waves define the front. These inherently opposing profiles are merged by longitudinal timber members connecting cityscape and landscape to create a ruled surface between them. To accomplish the long spans and cantilevers, complex geometry, continually varying elevations, and visually exposed structure, SGH selected glue?laminated timber (glulam) for the primary rafters and purlins based on its strength and stiffness, geometric flexibility, and aesthetics. The rafters, spaced at 2 m o.c., are each different in overall shape to form the desired roof profile. Our design employed long spans and cantilevers with very few columns. By discreetly locating columns where interior walls or slabs were close to the roof structure, SGH accentuated the architect’s desire for minimal support and openness at the exhibition spaces. SGH also designed an array of unique structural features, including cable trusses “embedded” in the roof geometry, exposed glulam columns, and an interior steel and concrete structure that seems to grow from the field of LED lights, providing the appearance of wispy grains.
Q: Can you mention one peculiarity of the structure that you designed? Include a short description and 1 or 2 images/photos.
A: Close collaboration and a holistic approach to architecture, engineering, and fabrication allowed us to realize the vision of the structure with its almost-floating, wavy, timber roof. Achieving a roof structure that seems to float above the pavilion’s “Land of Hope” was the main challenge. A timber grid system defined by a rafter-and-purlin solution forms the roof’s major and unique concept and geometry, and allowed for the desired two-directional long spans while maintaining a lightweight structure. The array of parallel-but-curved continuous rafters intersect with the longitudinal, mostly-straight discontinuous purlin members at regular intervals, to create an evolution of the diagrid, a three-dimensional orthogonal system with partial moment connections in each primary member axis. The timber structure utilizes bi-axial partial moment connections for each orthogonal member connection.
Q: How did you cope with the fast-track nature of the design process?
A: Early and close collaboration with the architect was essential for making this roof happen. Given the relatively rigid geometry to maintain, the most important challenge was to find the main structural strategy or system early in the game. After two or three weeks of discussions, we figured out some main ideas for the structural system, and determined that we could actually handle the given geometry. Then, we first evaluated the idea of using steel rafters at specific locations to pick up and connect to the purlins to form three-dimensional system. Shortly after, we saw the opportunity to connect with cable purlins that are on top of another, effectively creating a truss (vertical or inclined). Especially in the LED field, where we created a really long span without interruption or columns, there was an opportunity to put two major trusses. Developing these five trusses, and then recognizing they were actually inclined, was essential. Once the main structural system was identified, we were able to explore several structural variations of specific details by exchanging 3D models with the architect. This system allowed us to find relatively quickly a solution that reflects the architect’s original vision.
Q: Did you use any specific software for the design? Can you expand on how you interacted with the contractor(s) and helped expediting construction?
A: We used several 3D design software programs that allowed us to constantly feed our structural analysis models with the latest, fine-tuned geometry provided by the architect. These include SAP 2000, SAFE, Grasshopper, Rhinoceros, and several others. In addition, using these types of software was essential during construction. In fact, the exchange of 3D files between the architect, SGH, the local engineer, and the fabricator allowed us to resolve conflicts and fabricate unique timber structural shapes in a very short time frame. This basically allowed bypassing the entire review of shop drawings by making changes directly in the models. This would probably have not been possible 10-15 years ago, but was crucial to the success of the design idea and construction of the entire pavilion.